Coaxial bidirectional flow control and relief valves and hydrostat system containing the same

ABSTRACT

Coaxial bidirectional flow control and relief valves are provided especially suited for use in hydrostat systems containing a filter or filters interposed between a pump and a hydraulic motor, to direct flow in one direction through a filter, and to direct flow in the opposite direction to bypass the filter. The valves are space-saving, can be kept closed under low biasing forces, and can be made light in weight, affording a low-mass valve that can be moved in milliseconds from a closed to an open position. The valves can control not only bidirectional forward and reverse flow paths but also a relief path for flow to bypass a filter which is clogged.

[ Sept. 30, 1975 United States Patent 1191 Cooper Lhotcllier l37/493.6

FOREIGN PATENTS OR APPLICATIONS 1 1 COAXIAL BIDIRECTIONAL FLOW CONTROLAND RELIEF VALVES AND E ZE SYSTEM CONTAINING THE 1,299,86l [2/1972United Kingdom................. 137/493 [75] Inventor: Roydon B. Cooper,Locust Valley.

PI'I'DHII) ExaminerMartin P. Schwadron Assistant ExaminerRobert J.Miller [57] ABSTRACT Coaxial bidirectional flow control and reliefvalves are provided especially suited for use in hydrostat systemscontaining a filter or filters interposed between a 137/493; 210/130pump and a hydraulic motor, to direct flow in one di- Fl6K /04 rectionthrough a filter, and to direct flow in the oppo- 137/493, 493.6, 493.7,site direction to bypass the .filter. The valves are space-saving, canbe kept closed under low biasing forces, and can be made light inweight, affording a 521 US. [51] Int. [58] Field of Search..............

, 630.18, 115; 2lO/l30,

low-mass valve that can be moved in milliseconds from a closed to anopen position. The valves can con- [56] References Cited UNITED STATESPATENTS trol not only bidirectional forward and reverse flow 7/493 9 Xpaths but also a relief path for flow to bypass a fil r {37 3 UX iSCIOggfid.

2 i: 29 Claims, 9 Drawing Figures 2.423677 7/1947 Balogh 2,951,5009/1960 Hunter............. 2,987,311 6/1961 Schilling m .11.. 1112 76112/1963 TLHHIS et dI Flor F2\ L1 or L2 REVERSE RELIEF FLOW E PXKIT US.Patent Sept. 30,1975 Sheet20f6 3,908,693

L1 or L2 F1or F2 &

A'RE

RELIEF IlQVi REVERSE E -PXLIT US. Patent. Sept. 30,1975 Sheet4 of63,908,693

lllllll lllllllll US. Patent Sept. 30,1975 Sheet 5 of6 3,908,693

US. Patent Sept. 30,1975 Sheet 6 of6 3,908,693

a2 mm am am mm mm mm 30: mmmm mm mm mm wm COAXIAL BIDIRECTIONAL FLOWCONTROL AND RELIEF VALVES AND HYDROSTAT SYSTEM CONTAINING THE SAMEHydrostat systems are composed of a hydraulic pump and a hydraulic motorcoupled together in a closed fluid-flow loop or circuit to provide afluid drive for vehicles and to operate light and heavy-duty machinery,such as tractors and earth-moving equipment and paper mill machinery.The pump operates the motor by pumping the fluid to the motor whichreturns the fluid to the pump, and the motor in turn rotates an axle orother rotating member to drive the vehicle or machinery. Operation ineither direction can be obtained in the same system by control of thedirection of flow of fluid through the system, and the side of the motorto which the fluid is pumped. Fluid entering the motor from a firstdirection drives the motor in one direction, while fluid entering themotor from a second direction drives the motor in the oppositedirection. The motor thus can drive the vehicle or machinery in eitherdirection, according to the direction offlow of the fluid from the pumpto the motor, The fluid flow between the pump and the motor is normallyin a closed circuit through either of two fluid paths, ne path beingfollowed for clockwise operation and the other path being followed forcounterclockwise operation, andthe fluid paths enter opposite sides ofthe motor so as to drive it clockwise or counterclockwise, for operationin one direction or the other, which may be forward or in reverse. Thefluid paths are in a closed flow loop or circuit of the type shown inFlG. l, and each path carries forward or reverse flow, according to thedirection of flow through the required system for the desired operation.

The terms clockwise" and counterclockwise" are applied herein to thedirection of operation of the fluid drive; clockwise or righth'and flowoperates the drive in one direction, and counterclockwise or lefthandflow operates the drive in the opposite direction.

The terms forward and reverse are herein applied to the direction offlow of fluid through a given fluid path of the system between thepumpand the motor. Forward flow is from the pump to the motor, and

reverse flow is from the motor to the pump, in the same 4 fluid path.Flow through a filter in the filtering direction is also referred to asforward flow; flow bypassing the filter is referred to as reverse flow.

It is thus seen that forward" as applied to the direction of flow in thefluid patheoincideswith and refers to the flow direction required foreither clockwise or counterclockwise operation.

Because the system operates the drive by fluid flow,-

and because the wear of the moving parts tends min- As the filterremoves suspended material from the fluid, the filter tends to becomeclogged, and the flow through the filter diminishes, while thedifferential fluid pressure across the filter increases. Unless thefilter element is changed before it becomes fully clogged, the motor maybe starved, and operation may become erratic or slow. Hydrostat systemssometimes operate under quite high internal fluid pressures, of theorder of 5,000 psi and higher. At such high pressures, if clogging issubstantial, and the differential fluid pressure across the filterelement increases and approaches system pressure, the filter mayrupture, dumping its load of contaminants upon the motor and thepump.

It is accordingly desirable, should it be impossible to change thefilter element for some reason, to provide for a relief path for forwardflow bypassing the filter element. Such a relief path ensures that thecontaminated load on the filter will not be dumped on-the motor and pumpsuddenly, due to rupture of the filter under a higher'than normaldifferential pressure across the filter.

A hydrostat system provided with a filter requires some means forcontrolling reverse flow so that it does not pass through the filter. lfreverse flow were to pass through the filter, it would unload thecontaminants separated out by the filter, and carry them back to thepump and motor, with resultant destructive action on their movingparts-Consequently, it is-customary to provide a bidirectional valve inthe system which under forward flow directs fluid through one path, byway of the filter, and under reverse flow directs fluid through anotherpath, bypassing the filter.

The design of a bidirectional valve that will meet the pressure andrapid "flow reversal requirements of modern hydrostat systems has posednumerous problems. and the bidirectional valves heretofore availablehave not been fully satisfactory inmeeting the requirements. Many suchsystems require a high speed of reversal of the drive, within from 40 tomilliseconds. The bidirectional valves that have been usedare notcapable of responding so quickly, and consequently there is a time lagin the reversing, which is undesirable.

British patent No. 1,299,861 to-Fairey Aviation Ltd. describes a closedcircuit hydrostat system including bidirectional valves which aretypical of those that have been used. The valves are complex inconstruction, ex-

' pensive-to make, and give a slow response to change in troduce foreignparticles. bits of metal and other debris into the hydraulic fluidcirculating through the system, it is customary to provide a filter ineach'fluid path, to filter the fluid. and thus clean thefluid from anyparticles which might damage the moving parts'of the motor and pump. Thefilter is usually interposed to cleanthe fluid during forward flow fromthe pump to the motor. The filter can also be interposed to filter thefluid during flow from the motor to the pump. Introduction of the filterto filter flow in either flow direction in the system ensures that onlyclean fluid is supplied to the motor and the pump. 7 v

the direction of operation from forward to reverse and from reverse toforward. The bidirectional valves do not provide a flow path whichbypasses a clogged filter.

A separate relief valve on the filter is required for this purpose,further increasing'the complexity of the system.

Another difficulty posed by hydrostat systems, which must be met bybidirectional valves, is the necessity of passing full flow in eitherdirection immediately, to avoid starvation of the motor and/or the pump.This poses a design problem in any valve which is responsive to adifferential fluid pressure across the valve. The usual design of valve,such as the ball-and-poppet and the Belle'ville washer, is reponsive tothe fluid pressure differential across thevalve to provide a large valveopening at a high differential pressure, and a lesser valve opening at"a lower differential pressure. Once the valve has opened a little, thepressure differential across the valve decreases,'with the result thatit becomes impossible to open the valve any further. The

larger the flow required, the larger the valve element that is needed toexpose a large opening, and this increases the differential pressurerequired to open the valve. Moreover, the larger the valve, the largerthe mass of the material which has to be put in motion to open thevalve. For these reasons, the design of a bidirectional valve that iscapable of operating in a confined space, has a low mass and opensquickly to provide for full flow immediately upon change of flowdirection, has proved to be a complex and perplexing problem, which hasnot been resolved up until now.

In accordance with the invention, a coaxial bidirectional flow controland relief valve is provided, responsive by way of pressure actuatingsurfaces to a predetermined fluid pressure differential thereacross toopen or to close, and especially designed for use in hydrostat systems.The valve can be arranged to direct forward fluid flow through one path,and reverse fluid flow through another path. Thus, when used incombination with a filter element, it can be arranged to direct forwardflow through the filter element and reverse flow bypassing the filterelement. in a preferred embodiment, the valve can also be provided withpressure actuating surfaces arranged under a predetermined fluidpressure differential to open a third path during forward flow bypassingthe filter element, so that the valve controls flow through twodifferent fluid paths when the flow is in the forward direction, andflow through one fluid path (which can be the same as one of the twoforward fluid paths) when flow is in the reverse direction.

The valve is in coaxial tubular form, with first and second tubularvalve elements reciprocating between open and closed positions along alongitudinal axis. The valve can thus be inserted in line, or within thefluid line, utilizing a portion of its open central flow space to acceptthe reciprocating movement of the valve elements. The valve can also beinserted in the open space provided by the core of a tubular filterelement. The two reciprocating valve elements are each provided withpressure actuating surfaces but on opposite sides, so as to move one ofthe valve elements or the other between open and closed positions inresponse to fluid pressure differentials thereacross arising from flowof fluid in either direction through the fluid line. During forwardflow, the first reciprocating valve element under a force applied to itsforward pressure actuating surface above a predetermined first minimummoves into a first position in which it directs forward flow through onepath, such as through a filter. Upon reversal of flow, a pressureactuating surface responsive to reverse flow on the second reciprocatingvalve element receives reverse fluid pressure in a manner to move thesecond valve element into an open position, to direct flow throughanother path bypassing the filter element. The preferred embodiment hasa first reciprocating valve element which has a second position intowhich it moves under a fluid pressure applied to the forward pressureactuating surface above a higher second predetermined minimum, such aswhen the filter element is clogged, to open fluid path for relief flowbypassing the filter element. This fluid path optionally utilizes thesame bypass line opened by the valve element but now for relief flow inthe opposite direction.

The hydrostat system provided in accordance with the invention thuscomprises a pump; a motor; a fluid line operatively connecting the pumpwith the motor,

and arranged to carry fluid flow therebetween in either direction; afilter interposed in the fluid line between the pump and the motor, forfiltration of fluid either in forward flow from the pump to the motor orin reverse flow from the motor to the pump, and a coaxial bidirectionalflow control valve in fluid flow connection with the fluid line and withthe filter, and interconnected by at least one fluid line in series flowconnection with the filter, and at least one fluid line in parallel flowconnection with the filter, and controlling flow through the series andthe parallel lines, the valve directing fluid flow in one directionthrough the filter via the line is series with the filter, and fluidflow in the opposite direction bypassing the filter to the pump via theline in parallel with the filter, sensing and responding to a fluidpressure differential thereacross arising from the direction of fluidflow that is higher on the upstream than on the downstream side, todirect flow in one direction through the filter, and close the linebypassing the filter, and to direct reverse flow in the oppositedirection bypassing the filter, and close the line leading to thefilter.

The coaxial bidirectional flow control and relief valve in accordancewith the invention comprises, in combination, a tubular valve housing;first and second valve seats in the housing; first and second valveelements separately reciprocable within the valve housing towards andaway from the first and second valve seats, respectively, between closedand open positions; bias means urging the valve elements in onedirection; and a fluid pressure receiving surface operatively connectedto each valve element, urging the first valve element in one directiontowards or away from its valve seat, and urging the second valve elementin the other direction, towards or away from its valve seat; the biasingforce of the bias means being adjusted to resist movement of each valveelement in said direction towards or away from its valve seat underdifferential fluid pressure applied to the fluid pressure receivingsurface up to a predetermined minimum, and the valve element moving insaid direction towards or away from its valve seat and opening atdifferential fluid pressures applied to the fluid pressure receivingsurface above said minimum, one of the valve elements being responsiveto differential fluid pressure from one side of the valve, and the othervalve element being responsive to differential fluid pressure from theother side of the valve, so that the valve is arranged to open or closein response to differential fluid pressure applied from either directionof flow.

The coaxial bidirectional flow control valve of the invention can alsobe arranged to provide normal and relief flow in one direction throughdifferent paths, in addition to return flow in the other directionthrough a third path, which can be one of these two paths, usually therelief path. In this event, the second valve element is provided with afluid pressure receiving surface receiving differential pressure fromeither side of the valve, and the biasing force of the bias means isadjusted to resist movement of the valve element towards or away fromits valve seat under differential fluid pressure applied to the fluidpressure receiving surface from either side of the valve up to apredetermined minimum, which on the side of the valve to which the firstvalve element is responsive is higher than the predetermined minimum atwhich the first valve opens. There is accordingly a range of lowerdifferential pressures over which the first valve element is open, butnot the second, up to the predetermined minimum at which the secondvalve element opens, and a second range of higher differential pressuresat which both the first and second valve elements are open. If the firstelement controls flow through a normal passage, and the second valveelement controls flow through a relief passage, normal and relief floware provided for in one direction. Flow in the other direction throughthe relief passage is controlled by the second valve element.

This kind of valve is particularly useful in series flow and parallelflow with respect to a filter element, such as in a hydrostat system.The first valve element can control normal series flow through thefilter element, and the second valve element can control flow in thesame direction but bypassing the filter element at differential fluidpressures above a predetermined minimum reached when filter elementblockage seriously reduces flow. .The second valve element also providesparallel flow in the opposite direction bypassing the filter element, toavoid unloading the filter element during such flow.

In a preferred embodiment of the invention, the valve elements aretubular, and provide for normal fluid flow through the open center ofthe first tubular valve element. Reciprocation of the second valveelement opens or closes a flow passage extending laterally of the valveelement, through or at one end of the tubular valve housing, bypassingthe fluid-flow passage through the valve element. The first valveelement can be arranged to move reciprocably within the second valveelement between positions opening and closing the passage for fluid flowthrough the first valve element, and the first valve element isresponsive to differential fluid pressure arising from flow in onedirection, and the second valve element is responsive to differentialfluid pressure aris ing from fluid flow in the other direction, and, ifdesired, in the same direction as well.

By utilizing the open tubular passage of the valve elements for fluidflow, the valves of the invention become capable of passing larger fluidflow at lower pressure drops than other designs of valve.

An advantage of the tubular construction is that the valve elements canbe light in weight, and reciprocate very quickly between open and closedpositions in several milliseconds.

Sealing means can be provided between the valve elements and the valvehousing to prevent fluid leakage therebetween through the valve. Thesealing element is not essential, and a fluid-tight fit between thevalve elements and the tubular valve housing can also be employed, andis preferred, especially at high fluid pressures, such as may beencountered in hydrostat systems.

Since the valve elements are normally closed when there is no fluidflow, the valve also serves as an antidrain-back valve. If the valve isin series with a filter element when a filter element is being changed,the valve prevents loss of fluid from the line leading to the motor, andsince the pump is in effect a closed valve, there is no loss of fluidfrom the line leading to the pump. Consequently, the only fluid lostduring a filter change is the fluid in the filter bowl.

Preferred embodiments of the coaxial valve of the invention are shown inthe drawings, in which:

FIG. 1 is a flow diagram showing a hydrostat system having a pump and amotor connected by two fluid lines, each with a filter assembly and acoaxial valve of the invention in each line;

FIG. 2 is a detailed view in longitudinal section through a filterassembly of FIG. 1, including a filter bowl, a tubular filter elementwithin the bowl, and a coaxial valve in accordance with the inventiondisposed within the filter element, in which the reciprocable valveelements control flow through or bypassing the filter element underpredetermined conditions of differential fluid pressure arising fromfluid flow in either direction through the filter assembly, and showingboth valve elements in closed positions;

FIG. 3 is a cross-sectional view taken along the line 2-2 of FIG. 2, andlooking in the direction of the arrows;

FIG. 4 is a view in longitudinal section taken along the line 4-4through the filter assembly of FIG. 2, showing the first valve elementin the normally open position, for filtered flow through the filterelement, when the flow is in the normal forward direction, and thesecond valve element is closed;

FIG. 5 is a view in longitudinal section, taken along the line 55through the filter assembly of FIG. 2, showing the first valve elementclosed, and the second valve element in the open position to bypass thefilter element, when the flow is either in the forward direction, withthe filter clogged, or in the reverse direction;

FIG. 6 is a cross-section through the valve element of FIGS. 4 and 5taken along the line 6-6 and looking in the direction of the arrows;

FIG. 7 is a view in longitudinal section through another embodiment ofcoaxial valve of the invention, providing for forward and return flow,but not relief flow; and showing the valve position during forward flow;

FIG. 8 is another view of the valve of FIG. 7, showing the valveposition during return flow; and

FIG. 9 is a cross-section through the valve element of FIGS. 7 and 8,taken along the line 99 and looking in the direction of the arrows.

The tubular housing has an internal bearing surface or track along whichthe valve elements travel during their reciprocating movement betweenthe open and closed positions. The bearing surface or track can be aninternal wall of the housing, along which the valve elements can move.Alternatively, a bearing insert or sleeve can be placed within thehousing, to serve as the valve element track. Such a surface if porouswill be self-lubricating due to the fluid passing through the sys temfilling the pores of the surface or sleeve.

For convenience and ease of manufacture, the tubular housing and/or thetrack are cylindrical, and the valve elementsare also cylindrical, andcoaxial therewith. However, any other cross-sectional configuration canbe used, such as square, triangular, or polygonal. Configurations notround constrain the valve elements to reciprocating movement, andprevent rotation, which is desirable in some systems.

The valve elements have an external configuration matching the bearingsurface or track within the tubular housing, for reciprocating movementtherealong between their limiting positions. The length of movement ofthe valve elements is in no way critical, and the bearing surface ortrack is long enough to accommodate such movement.

Normally, although not necessarily, the valve elements are tubular, andeach has a central passage therethrough for normal fluid flow. In thisform, the relief valve is particularly adapted for use in a filterassembly, in which the valve can be placed within the internal core of atubular or cylindrical filter element, thus saving space. Such an opencentral passage can be closed off wholly or partially, as desired,according to the system requirements. It can for example be closed offby a check valve, which opens to fluid flow only in one directiontherethrough, to prevent backflow.

Each tubular valve element is provided with an annularpressure-receiving surface between two portions of differing diameterwhich receives fluid pressure on each side thereof, and thus registersdifferential fluid pressure thereacross. The valve element isoperatively connected to the pressure receiving surface in a manner tobe urged in one direction, towards either the open or the closedposition as desired. The pressure receiving surface should have adifferential pressurereceiving area sufficient to overcome the biasingforce of the bias means and move the valve element in this direction.

Such a pressure surface is usefully formed in a tubular valve element,as a ledge on the tube extending all or only part of the way around thetube, and leading to a portion of larger or lesser diameter. It is alsopossible to provide one or more projecting vanes or flanges along theperiphery of the valve elements. A sealing element or ring operativelyconnected to the-valve elements at their periphery can serve as apressure surface.

Normally, the valve elements are arranged to move in opposite directionsto an open position, under the impulse of the pressure receivingsurface, but they can be arranged to move in the same direction. Whenopening, the valve elements can expose the same or different passages,for bypass of a filter element, or for other purposes. The valve openingcan extend over all or part of the circumference of the valve elements,according to the flow required.

The exterior of the valve elements can be made to fit with a closeclearance against a bearing surface or track of the tubular housing, orthe external valve element of a coaxial pair. The clearance can besufficiently close so that a leak-tight seal is formed therebetween,preventing leakage past the relief valve.

It is also possible to interpose a sealing element between the exteriorof the valve element and the bearing surface or track. Such a sealingelement can be fixed to the wall of the tubular housing or to the valveelement; in the former it is stationary, and in the latter itreciprocates with the valve element. It has, however, been founddesirable to provide a sealing element which is not fixed to either, butwhich floats freely in the space between the valve element and thebearing surface or track of the tubular housing. The sealing element inthis case can slide or rotate within this space as the valve elementreciprocates along the track, reducing friction and thereby thedifferential pressure forces required to move the valve element. Thefloating sealing element can serve as the pressure receiving surface toreciprocate the valve element, even though it may move along the valveelement as it transmits reciprocating force thereto sufficient to drivethe element in one direction or the other.

One or several bias means is provided tending to move each valve elementtowards or away from its valve seat, and opposed to the direction ofmovement of the valve under the force applied by differential fluidpressure at the pressure receiving surface. A single means biasing bothvalve elements can bejused, or separate bias means for each valveelement. The bias means resists movement of the valve element towards oraway from its valve seat under differential fluid pressures up to apredetermined minimum; at higher differential fluid pressures, theforceapplied to the. pressure receiving surface exceeds the biasingforce of the bias means, and compels movement of the valve in theopposite direction. In one such direction, the valve is moved to aclosed position, and in the other such direction, the valve is moved tothe open position. Thus, the.

valve can be arranged to open or toclose under such predetermineddifferentialfluid pressure.

The bias means can ta-keany form. A compression or tension spring iseasily fitted in'the central passage of one tubular valve member such asbetween the two valve members without materially obstructing the openspace available for fluid flow. Magnetic elements can also be used,arranged either toattract or-to repel one another, one magnetic elementbeing movable with the valve element, and-one being a fixed location inthe tubular housing at which it attracts or repels the element towardsor away from the valve seat. In all forms, the bias means impelsmovement. of the valve element in a direction opposed to the directionof the application of the actuating differential fluid pressure on thepressure receiving'surface. A combination of spring bias and magneticbias means can also be used.

It is usually convenient to place'therelief flow passage atone end of orthrough the tubular housing, extending laterally to the valve element.If the former, one valve element can be arranged to move towards or awayfrom a valve seat at one end thereof. If the latter. the relief flowpassage is arranged to, pass directly through both the valve elementsandthe tubular housing, and is opened only upon the'registration ofapertures at predetermined reciprocable positions of the valve elementswith respect to the tubular housing.

The coaxial valves of the invention are particularly adapted for use inhydrostat systems to control flow or tocontrol bypass of filterassemblies, where as previously indicated the valve can be positionedwithin the internal core ofa tubular filter element. If the filterelement is retained within a filter housing, the tubular valve housingcan be attached tolthe filter housing, and the filter element attachedto the tubular valve housing. For example, one filter end .cap can bemade with a central aperture that fits snugly over the exterior of thetubular valve housing in a press fit, and a fluid-tight sealtherebetween. The coaxial valve thus can retain the filter element in adesired position in the housing, and the press fit makes it possible toquickly change filter elements without in any way interfering with theattachment of thecoaxial valve to the housing. Other arrangements arealso possible, however. For instance, the coaxial valve can be mountedand retained solely within the filter core, and attached or removed fromthe filter housing together with the filter element, the filter elementbeing mounted to the housing in conventional manner.

The coaxial valves of the invention can be made of any suitablematerials, such as plastic or metal, Stainless steel is a particularlydurable material of construction, suitable for most uses especially infilter ele- 'ments because of resistance to attack by fluids, and ispreferred both for the valve element and for the tubular valve housingand other components of the coaxial valve. It is, however, also suitableto make the coaxial valve of plastic, such as polytetrafluoroethylene,nylon, polycarbonates, phenolformaldehyde, urea formaldehyde, ormelamine-formaldehyde resins. It is also suitable to fabricate the valvehousing and valve element of stainless steel, and interpose a durableplastic sleeve or insert therebetween as a track, such as, for example,polytetrafluoroethylene or nylon.

A particularly advantageous feature of the coaxial valves of theinvention is that their construction makes it possible to use sheetmetal for the tubular housing and internal sleeve, and for the valveelements. This considerably simplifies their fabrication, and reducesmanufacturing costs, as compared to other types of valves in whichmachined, extruded, or cast components are necessary.

Specific embodiments of the invention are illustrated in the drawings,which will now be described.

The hydrostat system of FIG. 1 is a typical closedcircuit-flow-pathsystem, with a pump P and a motor M interconnected by two fluid lines L1and L2. Line L1 enters the motor in a position D1 to drive or rotate themotor in one direction and line L2 enters the motor in an oppositeposition D2 to drive or rotate the motor in the opposite direction. Inone direction, the motor drives the system forward via drive shaft S,rotating in one direction. In the opposite direction, the motor drivesthe system in reverse via drive shaft S rotating in the oppositedirection. Thus, fluid pumped by the pump P through the line L1 to themotor M drives the system in one direction, such as forward, and fluidpumped by the pump P through the line L2 to the motor M drives thesystem in the opposite direction, such as in reverse.

In each line L1 and L2 there is a filter F1 and F2, and a valve of theinvention V1 and V2. Two lines S1 and P1, S2 and P2, interconnect thevalves V1 and V2 in series and in parallel, respectively, with thefilters F1 and F2, so that flow proceeds through the filter F1, F2 orbypassing the filter, but not both. When flow is in the direction fromthe pump to the motor in either line L1 or L2, the flow is via seriesline S1, S2 through the filter, and when flow is in the direction fromthe motor to the pump in either line L1 or L2, the flow is via parallelline P1, P2 bypassing the filter. Since fluid proceeds from the pump tothe motor in one line and returns from the motor to the pump in theother line, the flow is either via Ll, S1, Fl to the motor, and via L2,P2, to the pump, or via L2, S2, F2 to the motor and via L1, P1 to thepump.

In operation, when flow proceeds forward, from the pump to the motor, inline Ll, the valve V1 in response to the resulting fluid pressuredifferential in the forward direction opens line S1 and flow proceedsvia the filter F1 to the motor M. Return flow via line L2 to valve V2causes valve V2 in response to the resulting fluid pressure differentialin the return direction to close line S2 and open line P2, so thatreturn flow proceeds via line P2, bypassing the filter F2 to the pump. i

A reversal of the pump reverses the direction of flow, so that flow nowproceeds from the pump via line L2 to the valve V2. In response to theresulting fluid pressure differential in the forward direction valve V2opens line S2 and closes line P2, so that flow proceeds via filter F2 tothe motor. Return flow via line L1 causes valve V1 in response to theresulting fluid pressure differential in the return direction to openline P1 and close line S1, so that flow bypasses filter F1 to the pump.

In the event that the filter F1 and/or F2 becomes clogged, the fluidpressure differential across the valve V1 and/or V2 increases, when flowproceeds in the forward direction, until eventually the fluid pressuredifferential is reached at which the valve opens the parallel line P1and/or P2 for flow bypassing the filter and relieving the fluid pressuredifferential.

The filter assemblies F1, F2 of FIG. 1, connected to lines L1 or L2, andshown in detail in FIGS. 2 to 6, each include a housing 1 having aninlet port 2 and an outlet port 3, communicating via chamber 4 acrossthe coaxial valves of the invention V1 or V2. The chamber 4 is generallycylindrical in configuration, with the inlet port opening into it at anangle of from the outlet port. The housing portion 6 defines the chamber4, and is open at the end of the chamber 4 opposite the outlet port. Theportion 6 of the housing terminates in a cylindrical threaded support 5for a bowl 9. The threads 7 of the support 5 engage mating threads 8 onthe inner wall of the bowl 9. An O-ring l0 and back-up ring 10' inrecess 11 in the bowl 9 provide a leak-tight seal between the bowl 9 andthe support 5.

The central portion of the bowl 9 has a through bore 13 serving as asocket for a differential pressure indicator 14, and a second side bore17 serving as an outlet or drain port in which is threadably mounted thedrain plug 18. Two O-rings'lS provide a leak-tight seal between thedifferential pressure indicator 14 and the bore 13. An O-ring 16 on acircumferential groove of the plug 18 provides a leak-tight seal betweenthe plug 18 and the bore 17. The heads of plug 18 and differentialpressure indicator 14 are hexagonal, to facilitate their insertion andremoval in the bores.

Centrally disposed in the open chamber 4 within housing portion 6 is afilter element 26 composed of a corrugated cylindrical filter 27,suitably of stainless steel wire mesh of suitable pore size, for example300 mesh, or a microporous multilayer element having a paper substratein accordance with U.S. Pat. No. 3,353,682, dated Nov. 21, 1967 to DavidB. Pall and Cyril A. Keedwell, supported on an internal core 30 ofcylindrical perforated stainless steel having through openings 32 forfluid flow, both filter and core being confined between end caps 28 and29. The ends of the filter element and core are sealed in a leak-tightmanner to the'end caps by the potting compound 31.

The end cap 29 has a central opening closed off by nipple 33 of the bowl9, which closes off the open central passage 35 within the core at thatend, sealed in a leak-tight manner by O-ring 34, captured by flange 41.The end cap 28 has a central opening 36, and is fitted with a flange 37surrounding the opening and defining an internal groove 38 within whichis captured an O- ring 39. The O-ring 39 provides a fluid-tight sealagainst the sleeve 40.

The sleeve 40 is a strip of stainless steel sheet pressed to form acentral tubular portion over which the filter element end cap 28 fits,and an outwardly-turned flanged portion 42 which is securely attached byscrews 49 between the ring 43 and plate 44 to the portion 45 of thehousing 1. The central portion of the sleeve 40 extends overasufficiently long reach so that the filter element is prevented frombeing withdrawn therefrom y when the bowl 9 is in place. shown in theFiguresv On the other hand, the filter element 26 is prevented frommoving too far in the other direction, towards the dependent portion 45of the housing, by the filter element stop 46, which is an outwardlyprojecting flange on an external sleeve 47 fitting over sleeve in apress fit.

As best seen in FIG. 3, the plate 44 has three fingers provided withopenings 51 through which are threaded screws 49 into threaded sockets49 of the dependent portion of the housing. Annular spacer rings 57 fitwithin recess 58 at the inner face of the housing portion 45, and haveholes 59 for the screws 49. The plate is held to the housing against therings 57 by the screws 49.

As best seen in FIG. 2, the dependent portion 45 of the housing has athrough bore 55 in fluid flow communication with the outlet port 3 andthe chamber 4 of the housing. The bore 55 is drilled out in a portion ofwider diameter constituting the recess 58. The plate 44 and ring 43 areattached to the housing across the recess 58 and the bore 55.

The sleeve 40 constitutes a tubular housing for the coaxialbidirectional flow control and relief valve V1 or V2 of the invention.best seen in FIGS. 4 to 6.

The valve has a first tubular valve element 50 movable therewithinbetween open and closed positions away from and towards a valve seat 52on cap 53. which closes off the open central passage 55 through thevalve and is attached to sleeve 54. The valve element 50 is normally inthe closed position. but opens under forward fluid pressure duringforward flow in line L1 or L2 from pump P to motor M through the filterelement 26, from inlet 2 to outlet 3. Both the tubu lar valve housingand the tubular valve element are made of stainless steel.

A second tubular valve element 70, which is also normally in the closedposition but opens under abnormal differential pressure under forwardflow through the filter element. or under reverse flow differentialpressure in the opposite direction. is retained within the sleeve 40 bythe ring 43. This element is free to move within the sleeve 40 betweenpositions towards and away from the housing portion 45, into and awayfrom sealing contact with valve seat 48 on the surface of housingportion 45, within recess 58.

The sleeve 40 at its bowl-end is provided with an inwardly-turned flange61 defining a central opening 62. The flange 61 serves as a seat for thespring-bias means 63, in this case a compression spring of temperedsteel. supporting a spring-retaining ring 64' which supports one end ofthe compression spring 63. The other end of the spring 63 bears againstthe flange of sleeve 54. To the other side of flange 60 is attached. asby welding. swaging. soldering. or brazing. the retaining ring 65, whichbears against the ledge 66 of valve element 70.

The ledge 66 of valve element leads to an end portion 71 of reduceddiameter. thus defining a space 72 between the exterior of the valveelement 70 and the interior of the tubular valve housing 40. Within thisspace is captured an O-ring 59, which is free to slide or rotate withinthe space 72, with movement of the valve element 70 in either direction.Normally. however. the O-ring is retained as shown in FIGS. 4 and 5against the exterior of the ledge 66 of the valve element 70.

The valve element 50 also has a ledge 57 leading to an end portion 58 ofreduced diameter at the valve seat 52. This enables the element end 58to clear the flange 56 of the cap 53, and seat on valve seat 52. On theinterior face of ledge 57 is supported one end of a second compressionspring 64, the other end of which is retained against the inside face ofretaining ring 65.

Thus. under the biasing force of springs 64. 63 each valve element 50.70, respectively. seats at one end 58, 71 against a valve seat 52, 48,respectively. at one limiting position. In the other limiting position.the valve element 50. 70 has moved away from the valve seat 52. 48,respectively. exposing a passage 75, 74, respectively. for fluid flowupon such movement. These movements of the valve elements 50, 70 arecontrolled by differential fluid pressure against the biasing forces ofthe springs 63, 64. respectively. as will now be seen.

The outside face of ledge 66 of the valve element 70 and the O-ring 59are exposed to fluid pressure on the pump side of the valve. in chamber4. in the forward flow direction from the pump to the motor. The innerface of ledge 66 is exposed to fluid pressure on the motor side of thevalve. in open central passages 35 and 68 of the filter element andvalve. respectively. During the forward flow from pump to motor. thefluid pressure in chamber 4 is greater than that in bore 55. and theresulting differential pressure across the valve tends to move the valveelement 70 away from the seat 48. Such movement is resisted by thespring 63. which biases the valve element towards that seat. up to apredetermined minimum forward differential fluid pressure. When theforce arising from differential fluid pressure applied at the O-ring 59and ledge 66 exceeds the biasing force of the spring. the valve element70 moves away from the seat 48. thereby exposing the passage 74 to fluidflow. bypassing the filter element 26. This occurs when fluid flowproceeds in the forward direction from pump P through line L1 or L2towards the motor M through chamber 4 through the filter element 26.towards the outlet port 3 via central passages 35 and 68.

The valve element 70 also is arranged to open during reverse fluid flowproceeding from motor M through line L1 or L2 to outlet port 3 and bore55 to inlet port 2. and thence to the pump. It is desired when reversefluid flow proceeds through line L1 or L2 that the filter element F1 orF2 be bypassed. so as to avoid unloading the contaminants on theupstream side of the filter element 27. Such opening of the valve V1 orV2 is accomplished due to return differential fluid pressure across thevalve under reverse fluid flow. The inner faces of cap 53 and flange 60of sleeve 54 are exposed to fluid pressure in passage 68. and the outerfaces of the cap 53 and flange 60 are exposed to fluid pressure inchamher 4. The valve thus can respond to reverse differential fluidpressure under reverse flow towards chamber 4 from outlet 3, tending toforce the cap 53 and sleeve 54 and with them spring 63 and valve element70 away from .the valve seat 48. Under reverse fluid flow. fluidpressure in space 68 within the valve element 50 against the ledge 57 isgreater than fluid pressure on the outer side of valve element 50against ledge 57. Com- 'pression spring 64 normally holds the valveelement 50 in the closed position. and there is no force tending to openvalve 50. which remains closed. Thus. fluid pressure in space 68 isexerted against cap 53 and ledge 60. The biasing force of the spring 63is quickly exceeded by the differential fluid pressure arising acrossthe cap 53 and ledge 60, and the sleeve 54 and valve element 70 with itmove from the closed to the open position, away from valve seat 48, thusopening passage 74 to reverse fluid flow, bypassing the filter element.

Operation of the filter assembly and coaxial valve is as follows: Underforward flow, fluid enters the filter housing I from line L1 or L2 viathe inlet port 2, proceeds into the chamber 4, and thence through thefilter element 26 towards the motor, while contaminants and othersuspended solid materials are filtered out. The filtered fluid passesthrough the perforations 32 in the core 30, into the open passage 35 inthe interior of the core. The valve element 50, under forwarddifferential fluid pressure across the valve sufficient to exceed thebias force of spring 64, has moved away from valve seat 52, exposingpassage 75, and so flow proceeds from passage 35 through the passage 75between cap 53 and valve element 50 into the interior passage 68 throughthe valve element 50 and bore 55 to the outlet port 3 of the housing 1,and thence via line L1 or L2 to the motor.

As the flow continues, the filter element 26 gradually becomes loadedwith contaminants, and flow through the element is decreased. As thefilter element increasingly becomes incapable of passing normal fluidflow, the pressure differential across the filter element increases,with the result that the differential pressure on the O-ring 59 ledge 66of the valve element 70 in creases. At a differential pressure justbefore the valve crack-open differential pressure, the differentialpressure indicator I4 is triggered, and a signal given. The filterelement should then be changed. If it is not, the differential pressurecontinues to increase. Eventually the differential pressure againstO-ring 59 and ledge 66 reaches the crack-open pressure, and exceeds theresistance of the spring 63, pushing the valve element 70 away from seat48 and opening passage 74 for relief flow bypassing the filter element26, and this flow continues until the condition is corrected or thefilter element is changed. Valve element 50 remains open, if thedifferential pressure thereacross remains high enough to exceed thebiasing force of spring 64; if it does not, it closes, but bypassingflow via passage 74 and open valve 70 can continue, despite this.

In the event that the filter element 26 is to be replaced, flow is shutoff. The bowl 9 can then be withdrawn on the threads 7, 8 exposing thefilter element 26. The valve elements 50, 70, are closed, so drain backof fluid from line L1, L2 to the motor is prevented. Since the filterelement 26 is attached over the sleeve 40 in a press fit, it is easilyslipped off, and a fresh filter element substituted. The bowl 9 can thenbe replaced. In order to avoid introduction of air into the system, itmay be desirabl'eto fill the bowl 9 with fluid before replacing it.

In the event that fluid flow through the line L1 or L2 and filterassembly F1 or F2 is reversed, the coaxial valve ensures that reverseflow will not proceed through and unload the filter element 26.Immediately that reverse flow begins, the differential fluid pressureacross valve element 50 is reversed, and now bears on ledge 57 to drivethe element 50 closed, against valve seat 52. This closes off the flowthrough passage 75, whereupon the reverse flow differential fluidpressure across the cap 53, sleeve 54 and valve element 70 increases, toa force sufficient to overcome the biasing force of spring 63, andsleeve 54 and with it valve element 70 are then moved away from valveseat 48,

opening passage 74 to return flow bypassing the filter 26 into chamber24.

This situation continues while reverse flow continues. When reverse flowceases, the differential fluid pressure holding the cap 53, sleeve 54and valve element open drops to Zero, and spring 63 brings the valveelement 70 to valve seat 48, closing the valve. Valve element 50continues to be closed, since there is no flow. The coaxial valve is nowready to accept forward flow. If forward flow is resumed, valve element50 opens, as before.

It will be apparent from FIGS. 4 and 5 that when the valve element 70moves to the open position, it carries with it valve element 50, sincering 65 bears against the end 76 of the element 50. However, valve 70moves only a relatively short distance axially to open (see FIG. 5 andat the open position ring 65 does not quite drive valve element 50 tothe closed position; it remains partially open, to accept such filteredflow as may pass through the filter element 2 6. However, if desired,the valve element 50 can be arranged to be closed when valve element 70is open, merely by making the distance traveled by each valve the same,between open and closed positions. Alternatively, by shortening thevalve element 50, it can be made so as not to be engaged by ring 65,whatever the position of valve element 50.

The embodiment of coaxial bidirectional flow control valve of theinvention shown in FIGS. 7 to 9 has a sleeve 80 which constitutes atubular housing.

A first tubular valve element 81 is movable within a second tubularvalve element 90, coaxial therewith, between open and closed positionsaway from and towards a valve seat 82 on cap 83, which closed off theopen central passage 85 through the valve, and is attached to the secondvalve element 90. The valve element 81 is normally in the closedposition shown in FIG. 8, but opens under forward differential fluidpressure across the valve during forward flow applied at the ledge 84 ofthe valve element. Both the tubular valve housing 80 and the tubularvalve elements 81, are made of heat-treated steel.

The second tubular valve element 90 is normally in the closed positionshown in FIG. 7, but opens under reverse differential pressure duringreverse flow. The valve element is retained withinthe sleeve 80 by theflange 96, seating on ledge 97 of the sleeve when the valve is in theopen position shown in FIG. 8. This element is free to move within thesleeve 80 between posi tions towards and away from the housing portion98, into and away from sealing contact with valve seat 99 on the surfaceof housing portion 98.

The ledge 91 of valve element 90 leads to an end portion 92 of reducedexternaldiameter, thus defining a space 93 between the exterior of thevalve element 90 and the interior of the tubular valve housing 80.Within this space is fitted a coil spring 94, which is retained thereinby the lock ring 95. The spring 94 biases the valve element 90 againstthe seat 99 on the housing portion 98.

The valve element 81 has a ledge 84 leading to an end portion 86 ofreduced diameter seating at the valve seat 82. The interior face ofledge 84 serves as a retain ing support for the coil spring 87, theother end of which is retained against the inside face of the flangedring 88, seated on retaining ring 89 seated in valve element 90. Thespring 87 retains the valve element 81 against the valve seat 82.

Thus, under the biasing force of springs 87, 94 each valve element 81,90, respectively, seats at one end against a valve seat 82, 99,respectively, at one limiting position. In the other limiting position,the valve element 81, 90 is moved away from the valve seat 89, 99,respectively, exposing a passage 100, 101, respectively, for fluid flov.upon such movement. These movements of the valve element 81, 90 arecontrolled by differential fluid pressure against the biasing forces ofthe springs 87, 94, respectively.

The valve elements 81, 90 are coaxial, and each is reciprocable withinand over the other, respectively, independently and without regard towhether the other is stationary. The two elements are fitted snuglytogether in a leak-tight fit, so that no supplemental sealing isnecessary, even at relatively high differential fluid pressuresthereacross. In like manner, valve element 90 is reciprocable withinsleeve 80, and is fitted snugly therewithin in a leak-tight fit.

The sleeve 80 is retained in a fixed position relative to the housingportion 78 by the retaining ring 79.

The outside face of ledge 84 of valve element 81 is exposed to fluidpressure on the one side, for example, the pump side, of the valve, andthe inner face of the ledge is exposed to fluid pressure on the otherside, for example, the motor side, of the valve.

The inside face of cap 83 and the outside face of flange 96 of the valveelement 90 are exposed to fluid pressure on the other side, the motorside, of the valve. The outer face of ledge 91 is exposed to fluidpressure on the first side of the valve. During forward flow, from pumpto motor, the fluid pressure on the pump side of valve 81 is greaterthan on the motor side, and the resulting differential pressure acrossthe valve applied at ledge 84 tends to move the valve element 81 awayfrom the seat 82. Such movement is resisted by the spring 87, whichbiases the valve element towards that seat, up to a predeterminedminimum forward differential fluid pressure. When the force arising'fromdifferential fluid pressure applied at the ledge 84 exceeds the biasingforce of the spring, the valve element 81 moves away from the seat 82,thereby exposing the passage 100 to fluid flow. This position of thevalve continues while fluid flow proceeds in the forward direction.

The valve element 90 is arranged to open during re verse fluid flow, forexample, from the motor proceeding to the pump. During reverse flow, theinside face of cap 83 and the outside face of flange 96 are exposed toreverse fluid pressure on the motor side of the valve, and the outerface of the ledge 91 is exposed to fluid pressure on the pump side ofthe valve. The valve 90 thus can respond to return differential fluidpressure under reverse flow tending to force the valve element 90 awayfrom the valve seat 99. Under reverse fluid flow, fluid pressure againstthe inside face of cap 83 and the ledge 96 is greater than fluidpressure on the other side of the valve element 90 against ledge 91.Compression spring 94 normally holds the valve ele ment 90 in the closedposition, and there is no force tending to open valve 81 which remainsclosed. Thus. reverse fluid pressure is exerted against cap 83 and ledge96. The biasing force of the spring 94 is quickly exceeded by thedifferential fluid pressure arising across the cap 83 and ledge 96, andthe valve element 90 moves from the closed to the open position awayfrom valve seat 99, thus opening passage 101 to reverse fluid flow forthe purpose, for example, of bypassing the filter element.

This valve has no provision for relief bypass flow.

Operation of this coaxial valve is as follows: Under forward flow, fluidapproaches the valve element 81, and encounters ledge 84 while the valveelement is in the closed position shown in FIG. 8. The valve element 81,under forward differential fluid pressure across the valve applied atthe ledge 84 sufficient to exceed the bias force of spring87, is movedaway from valve seat 82, into the position shown in FIG. 7, exposingpassage 100, and so flow proceeds through the passage within valveelement 81, past the valve. While flow continues in the forwarddirection, valve remains in the closed position shown in FIG. 7.

When fluid flow through the valve is reversed, the coaxial valve ensuresthat reverse flow will not proceed through the passages 85, 100, butproceeds via passage 101 through another flow path. Immediately thatreverse flow begins, the differential fluid pressure across valve 81 isreversed, and now bears on the inside face of ledge 84-50 that there isno force to hold element 81 open and the spring 87 therefore drives theelement 81 closed, against valve seat 82. This closes off the flowthrough passage 85, whereupon the reverse flow differential fluidpressure across the inside face of cap 83 and the outside face of flange96 increases, to a force sufficient to overcome the biasing force ofspring 94, and valve element 90 is then moved away from valve seat 99,opening passage 101 to reverse flow bypassing the line communicatingwith passage [00. The valve position is then as shown in FIG. 8. I

This situation continues while reverse flow continues. When reverse flowceases, the differential fluid pressure holding the cap 83, flange 96,of valve element 90 open drops to zero, and spring 94 brings the valveelement 90 to valve seat 99 closing the valve. Valve element 81continues to be closed as shown in FIG. 8, since there is no flow. Thecoaxial valve is now ready to accept forward flow. If forward flow isresumed, valve element 81 opens, as before.

It will be apparent from FIGS. 7 and 8 that when the valve element 90moves to the open position, it carries with it valve element 81, sincering 89 bears against the ring 88 and cap 83 is attached to the element90. However, valve element 81 is not openedby such' movement of valveelement 90.

While the embodiments shown in the drawings each have valve elementsheld normally in the closed position by compression springs, one or bothof the valve elements can also be held normally in the open position, bytension springs, and closed under a predetermined differential pressurethereacross, exceeding a predetermined minimum. The valve can thus bemade to close, to prevent pressure surges from either direction frompassing through, as in reverse flow, for example, but otherwise allowflow freely in either direction. The valve can also be arranged to allownormal flow through one line in either or both directions. The valve canalso be arranged to allow normal flow through one line in either or bothdirections, and divert pressure surges in either or both directions toanother bypass line. I

The valveis thus quite versatile'in protecting a filter element fromdamage under flow from either direction or unloading from the otherdirection, and in controlling bypass flow past a filter element fromeither direction, under predetermined differential fluid pressures.

The valve of the invention is useful as a normally closed dual checkvalve in any system. Pressure relief path flow control from eitherdirection can be provided by constructing either or both valve elementsso as to be responsive to differential pressures from either di rection,at first and second stages, each above a predetermined minimum, andindependently of each other.

Having regard to the foregoing disclosure, the following is claimed asthe inventive and patentablc embodiments thereof:

1. A coaxial flow-directional control valve responsive to differentialfluid pressure thereacross arising from flow of fluid in eitherdirection through a flow passage, to direct flow through one of twodifferent valve flow passages and thereby provide a bypass flowfunction, comprising, in combination, a tubular valve housing; first andsecond valve seats in the housing; first and second coaxial tubularvalve elements nested concentrically and separately reciproeable withinthe valve housing towards and away from the first and second valveseats, respectively, between closed and open positions, to close andopen first and second valve flow passages, respectively, of which oneprovides a bypass function; a fluid-pressure receiving surfaceoperatively connected to each valve element, urging the first valveelement in a first direction towards or away from its valve seat, andthe second valve element in a second direction, towards or away from itsvalve seat; bias means urging each valve element in the oppositedirection, the biasing force of the bias means being adjusted to resistmovement of each valve element in said direction away from or towardsits valve seat except under differential fluid pressure arising fromflow and applied to the fluid pressure receiving surface from the firstor second flow direction, and each of the valve elements moving in saiddirection towards or away from its valve seat and opening when flowbegins from that direction and closing when flow stops from thatdirection, the first valve element being responsive to differentialfluid pressure arising from flow in the first direction from one side ofthe valve, and the second valve element being responsive to differentialfluid pressure arising from flow in the second direction from the otherside of the valve, so that the valve is arranged to open or close inresponse to differential fluid pressure arising and applied from eitherdirection of flow, and to direct such flow through one of the first andsecond valve flow passages according to flow direction.

2. A coaxial flow-directional control valve according to claim 1, inwhich the valve elements are light in weight and reciprocate betweenopen and closed positions within several milliseconds.

3. A coaxial flow-directional control valve according to claim it, inwhich one of the valve elements is arranged to open under differentialfluid pressure from either side of the valve to control flow in areverse direction through one of the first and second valve flowpassages.

4. A coaxial flow-directional control valve according to claim l, inwhich the tubular valve housing has an internal wall along which thevalve elements reciprocate between open and closed positions.

5. A coaxial flow-directional control valve according to claim 4,, inwhich the exterior of the valve element and the internal wall have aclose enough clearance to define a fluid-tight seal therebetween.

6. A coaxial flow-directional control valve according to claim 4, inwhich at least a portion of the internal wall and the exterior of thevalve element are spaced apart; and a sealing means is disposed withinthat space, providing a fluid-tight seal between the wall and the valveelement.

7. A coaxial flow-directional control valve in accordance with claim 1,in which the bias means is a coil spring.

8. A coaxial flowdirectional control valve in accordance with claim 1,in which the inner tubular valve element has an open central passage forflow of fluid therethrough.

9. A coaxial flow-directional control valve in accordance with claim 8,in which each tubular valve element has portions of relatively large andrelatively small diameter connected by a ledge providing the fluidpressure-receiving surface.

10. A coaxial flow-directional control valve in accordance with claim 9,in which the portion of lesser diameter of the outer valve elementdefines a space with the tubular valve housing, within which is disposeda sealing means movable with reciprocation of the valve element withinthe space, and the portion of lesser diameter of the inner valve elementdefines a space with the outer valve element, within which is disposed asealing means movable with reciprocation of the valve element within thespace.

1 l. A coaxial flow-directional control valve in accordance with claim1, in which the bias means urges each valve element towards its valveseat, and the fluid pressure-receiving surface is operatively connectedto the valve element to urge it away from its valve seat.

12. A coaxial flow-directional control valve in accordance with claim11, in which each fluid pressurereceiving surface of each valve elementis exposed to fluid pressure both upstream and downstream of the valve,and a fluid pressure differential therebetween arising from flow fromeither direction moves at least one valve element, overcoming the biasforce of the bias means.

13. A coaxial flow-directional control valve in accordance with claim11, having a tubular sleeve disposed between the first and second valveelements, and movable with the outer valve element, the inner valveelement being reciproeable within the tubular sleeve between closed andopen positions towards and away from a valve seat connected to andmovable with the tubular sleeve.

14. A coaxial flow-directional control valve in accordance with claim13, in which the inner valve element is arranged in its closed positionto close off a central fluid passage through the inner valve element,and in its open position to open the fluid passage through the innervalve element.

15. A coaxial flow-directional control valve in accordance with claim14, in which the tubular sleeve has a cap closing off one end, and theinner valve element seats against the cap to close the fluid passagethrough the inner valve element.

16. A coaxial flow directional control valve responsive to differentialfluid pressure thereacross arising from flow of fluid in eitherdirection through a flow passage, to direct flow in one of at leastthree flow paths through one of two different valve flow passages,

and thereby provide a bypass flow function in one of at least three flowpaths, at least one valve flow passage receiving flow in both forwardand reverse flow directions, according to the position of the controlvalve, comprising, in combination, a tubular valve housing; first andsecond valve seats in the housing; first and second coaxial valveelements separately reciprocable within the valve housing towards andaway from the first and second valve seats, respectively, between closedand open positions, to close and open first and second valve flowpassages, respectively, of which one provides a bypass function; afluid-pressure receiving surface operatively connected to each valveelement, urging .the first valve element in a first direction towards oraway from its valve seat and thereby control flow in a first flow path,and the second valve element in a second direction, towards or away fromits valve seat and thereby control flow in a second flow path; one ofthe valve elements having a fluid-pressure receiving surface receivingpressure from either side of the valve element and being arranged toopen under differential fluid pressure from either side of the valve tocontrol flow in either direction through one of the first and secondvalve flow passages and thereby in addition control flow in a third flowpath; bias means urging each valve element in the opposite direction,the biasing force of the bias means being adjusted to resist movement ofeach valve element in said direction away from or towards its valve seatexcept under differential fluid pressure arising from flow and appliedto thc fluid pressure receiving surface from the first or second flowdirection, and each of the valve elements moving in said directiontowards or away from its valve seat and opening when flow begins fromthat direction and closing when flow stops from that direction, thefirst valve element being responsive to differential fluid pressurearising from flow in the first direction from one side of the valve, thesecond valve element being responsive to differential fluid pressurearising from flow in the sec ond direction from the other side of thevalve so that the valve is arranged to open or close in response todifferential fluid pressure arising and applied from either direction offlow and to direct such flow in one of the first and second flow pathsthrough one of the first and second valve flow passages according toflow direction, and one of the first and second valve elements inaddition being responsive to differential fluid pressure arising fromflow from the opposite direction to direct flow in the third flow paththrough one of the first and second valve flow passages in the oppositedirection.

17. A coaxial flow-directional control valve accordingto claim 16, inwhich the valve elements are light in weight and reciprocate betweenopen and closed positions within several milliseconds.

18. A coaxial flow-directional control valve according to claim 16, inwhich the tubular valve housing has an internal wall along which thevalve elements reciprocate between open and closed positions.

19. A coaxial flow-directional control valve according to claim 18, inwhich the exterior of the valve ele ment and the internal wall have aclose enough clearance to define a fluid-tight seal therebetween.

20. A coaxial flow-directional control valve according to claim 19, inwhich at least a portion of the internal wall and the exterior of thevalve element are spaced apart; and a sealing means is disposed withinthat space, providing a fluid-tight seal between the wall and the valveelement.

2]. A coaxial flowdirectional control valve in accordance with claim 19in which the bias means is a coil spring.

22. A coaxial flow-directional control valve in accordance with claim16, in which the first and second coaxial valve elements are tubular andconcentric, and the inner tubular valve element has an open centralpassage for flow of fluid therethrough.

23. A coaxial flow-directional control valve in accordance with claim22, in which each tubular valve element has portions of relatively largeand relatively small diameter connected by a ledge providing the fluidpressure-receiving surface.

24. A coaxial flow-directional control valve in accordance with claim23, in which the portion of lesser diameter of the outer valve elementdefines a space with the tubular valve housing, within which is disposeda sealing means movable with reciprocation of the valve element withinthe space, and the portion of lesser diameter of the inner valve elementdefines a space with the outer valve element, within which is disposed asealing means movable with reciprocation of the valve element within thespace.

25. A coaxial flow directional control valve in accordance with claim 16in which the bias means urges each valve element towards its valve seat,and the fluid pressure-receiving surface is operatively connected to thevalve element to urge it away from its valve seat.

26. A coaxial flow directional control valve in accordance with claim25, in which each fluid pressurereceiving surface of each valve elementis exposed to fluid pressure both upstream and downstream of the valve,and a fluid pressure differential therebetween arising from flow fromeither direction moves at least one valve element, overcoming the biasforce of the bias means.

27. A coaxial flow-directional control valve in accordance with claim22, having a tubular sleeve disposed between the first and second valveelements, and movable with the outer valve element, the inner valveelement being reciprocable within the tubular sleeve between closed andopen positions towards and away from a valve seat connected to andmovable with the tubular sleeve.

28. A coaxial flow-directional control valve in accordance with claim27, in which the inner valve element is arranged in its closed positionto close off a central fluid passage through the inner valve element,and in its open position to open the fluid passage through the innervalve element.

'29. A coaxial flow-directional control valve in accordance with claim28, in which the tubular sleeve has a closing off one end, and the innervalve element seats against the cap to close the fluid passage throughthe inner valve element.

1. A coaxial flow-directional control valve responsive to differentialfluid pressure thereacross arising from flow of fluid in eitherdirection through a flow passage, to direct flow through one of twodifferent valve flow passages and thereby provide a bypass flowfunction, comprising, in combination, a tubular valve housing; first andsecond valve seats in the housing; first and second coaxial tubularvalve elements nested concentrically and separately reciprocable withinthe valve housing towards and away from the first and second valveseats, respectively, between closed and open positions, to close andopen first and second valve flow passages, respectively, of which oneprovides a bypass function; a fluid-pressure receiving surfaceoperatively connected to each valve element, urging the first valveelement in a first direction towards or away from its valve seat, andthe second valve element in a second direction, towards or away from itsvalve seat; bias means urging each valve element in the oppositedirection, the biasing force of the bias means being adjusted to resistmovement of each valve element in said direction away from or towardsits valve seat except under differential fluid pressure arising fromflow and applied to the fluid pressure receiving surface from the firstor second flow direction, and each of the valve elements moving in saiddirection towards or away from its valve seat and opening when flowbegins from that direction and closing when flow stops from thatdirection, the first valve element being responsive to differentialfluid pressure arising from flow in the first direction from one side ofthe valve, and the second valve element being responsive to differentialfluid pressure arising from flow in the second direction from the otherside of the valve, so that the valve is arranged to open or close inresponse to differential fluid pressure arising and applied from eitherdirection of flow, and to direct such flow through one of the first andsecond valve flow passages according to flow direction.
 2. A coaxialflow-directional control valve according to claim 1, in which the valveelements are light in weight and reciprocate between open and closedpositions within several milliseconds.
 3. A coaxial flow-directionalcontrol valve according to claim 1, in which one of the valve elementsis arranged to open under differential fluid pressure from either sideof the valve to control flow in a reverse direction through one of thefirst and second valve flow passages.
 4. A coaxial flow-directionalcontrol valve according to claim 1, in which the tubular valve housinghas an internal wall along which the valve elements reciprocate betweenopen and closed positions.
 5. A coaxial flow-directional control valveaccording to claim 4, in which the exterior of the valve element and theinternal wall have a close enough clearance to define a fluid-tight sealtherebetween.
 6. A coaxial flow-directional control valve according toclaim 4, in which at least a portion of the internal wall and theexterior of the valve element are spaced apart; and a sealing means isdisposed within that space, providing a fluid-tight seal between thewall and the valve element.
 7. A coaxial flow-directional control valvein accordance with claim 1, in which the bias means is a coil spring. 8.A coaxial flow-directional control valve in accordance with claim 1, inwhich the inner tubular valve element has an open central passage forflow of fluid therethrough.
 9. A coaxial flow-directional control valvein accordance with claim 8, in which each tubular valve element hasportions of relatively large and relatively small diameter connected bya ledge providing the fluid pressure-receiving surface.
 10. A coaxialflow-directional control valve in accordance with claim 9, in which theportion of lesser diameter of the outer valve element defines a spacewith the tubular valve housing, within which is disposed a sealing meansmovable with reciprocation of the valve element within the space, andthe portion of lesser diameter of the inner valve element defines aspace with the outer valve element, within which is disposed a sealingmeans movable with reciprocation of the valve element within the space.11. A coaxial flow-directional control valve in accordance with claim 1,in which the bias means urges each valve element towards its valve seat,and the fluid pressure-receiving surface is operatively connected to thevalve element to urge it away from its valve seat.
 12. A coaxialflow-directional control valve in accordance with claim 11, in whicheach fluid pressure-receiving surface of each valve element is exposedto fluid pressure both upstream and downstream of the valve, and a fluidpressure differential therebetween arising from flow from eitherdirection moves at least one valve element, overcoming the bias force ofthe bias means.
 13. A coaxial flow-directional control valve inaccordance with claim 1, having a tubular sleeve disposed between thefirst and second valve elements, and movable with the outer valveelement, the inner valve element being reciprocable within the tubularsleeve between closed and open positions towards and away from a valveseat connected to and movable with the tubular sleeve.
 14. A coaxialflow-directional control valve in accordance with claim 13, in which theinner valve element is arranged in its closed position to close off acentral fluid passage through the inner valve element, and in its openposition to open the fluid passage through the inner valve element. 15.A coaxial flow-directIonal control valve in accordance with claim 14, inwhich the tubular sleeve has a cap closing off one end, and the innervalve element seats against the cap to close the fluid passage throughthe inner valve element.
 16. A coaxial flow directional control valveresponsive to differential fluid pressure thereacross arising from flowof fluid in either direction through a flow passage, to direct flow inone of at least three flow paths through one of two different valve flowpassages, and thereby provide a bypass flow function in one of at leastthree flow paths, at least one valve flow passage receiving flow in bothforward and reverse flow directions, according to the position of thecontrol valve, comprising, in combination, a tubular valve housing;first and second valve seats in the housing; first and second coaxialvalve elements separately reciprocable within the valve housing towardsand away from the first and second valve seats, respectively, betweenclosed and open positions, to close and open first and second valve flowpassages, respectively, of which one provides a bypass function; afluid-pressure receiving surface operatively connected to each valveelement, urging the first valve element in a first direction towards oraway from its valve seat and thereby control flow in a first flow path,and the second valve element in a second direction, towards or away fromits valve seat and thereby control flow in a second flow path; one ofthe valve elements having a fluid-pressure receiving surface receivingpressure from either side of the valve element and being arranged toopen under differential fluid pressure from either side of the valve tocontrol flow in either direction through one of the first and secondvalve flow passages and thereby in addition control flow in a third flowpath; bias means urging each valve element in the opposite direction,the biasing force of the bias means being adjusted to resist movement ofeach valve element in said direction away from or towards its valve seatexcept under differential fluid pressure arising from flow and appliedto the fluid pressure receiving surface from the first or second flowdirection, and each of the valve elements moving in said directiontowards or away from its valve seat and opening when flow begins fromthat direction and closing when flow stops from that direction, thefirst valve element being responsive to differential fluid pressurearising from flow in the first direction from one side of the valve, thesecond valve element being responsive to differential fluid pressurearising from flow in the second direction from the other side of thevalve so that the valve is arranged to open or close in response todifferential fluid pressure arising and applied from either direction offlow and to direct such flow in one of the first and second flow pathsthrough one of the first and second valve flow passages according toflow direction, and one of the first and second valve elements inaddition being responsive to differential fluid pressure arising fromflow from the opposite direction to direct flow in the third flow paththrough one of the first and second valve flow passages in the oppositedirection.
 17. A coaxial flow-directional control valve according toclaim 16, in which the valve elements are light in weight andreciprocate between open and closed positions within severalmilliseconds.
 18. A coaxial flow-directional control valve according toclaim 16, in which the tubular valve housing has an internal wall alongwhich the valve elements reciprocate between open and closed positions.19. A coaxial flow-directional control valve according to claim 18, inwhich the exterior of the valve element and the internal wall have aclose enough clearance to define a fluid-tight seal therebetween.
 20. Acoaxial flow-directional control valve according to claim 19, in whichat least a portion of the internal wall and the exterior of the valveelement are spaced apart; and a sealing mEans is disposed within thatspace, providing a fluid-tight seal between the wall and the valveelement.
 21. A coaxial flow-directional control valve in accordance withclaim 19 in which the bias means is a coil spring.
 22. A coaxialflow-directional control valve in accordance with claim 16, in which thefirst and second coaxial valve elements are tubular and concentric, andthe inner tubular valve element has an open central passage for flow offluid therethrough.
 23. A coaxial flow-directional control valve inaccordance with claim 22, in which each tubular valve element hasportions of relatively large and relatively small diameter connected bya ledge providing the fluid pressure-receiving surface.
 24. A coaxialflow-directional control valve in accordance with claim 23, in which theportion of lesser diameter of the outer valve element defines a spacewith the tubular valve housing, within which is disposed a sealing meansmovable with reciprocation of the valve element within the space, andthe portion of lesser diameter of the inner valve element defines aspace with the outer valve element, within which is disposed a sealingmeans movable with reciprocation of the valve element within the space.25. A coaxial flow directional control valve in accordance with claim 16in which the bias means urges each valve element towards its valve seat,and the fluid pressure-receiving surface is operatively connected to thevalve element to urge it away from its valve seat.
 26. A coaxial flowdirectional control valve in accordance with claim 25, in which eachfluid pressure-receiving surface of each valve element is exposed tofluid pressure both upstream and downstream of the valve, and a fluidpressure differential therebetween arising from flow from eitherdirection moves at least one valve element, overcoming the bias force ofthe bias means.
 27. A coaxial flow-directional control valve inaccordance with claim 22, having a tubular sleeve disposed between thefirst and second valve elements, and movable with the outer valveelement, the inner valve element being reciprocable within the tubularsleeve between closed and open positions towards and away from a valveseat connected to and movable with the tubular sleeve.
 28. A coaxialflow-directional control valve in accordance with claim 27, in which theinner valve element is arranged in its closed position to close off acentral fluid passage through the inner valve element, and in its openposition to open the fluid passage through the inner valve element. 29.A coaxial flow-directional control valve in accordance with claim 28, inwhich the tubular sleeve has a cap closing off one end, and the innervalve element seats against the cap to close the fluid passage throughthe inner valve element.